Theory of MAS DNP
Even if the discovery of DNP (at 0.003 T) is not exactly recent, the (deep and correct) understanding of the mechanisms at work during polarization transfer at high magnetic field under (rapid) sample spinning is not fully achieved yet. It is thus essential to carry out work in this direction with the goal to improve the understanding of the mechanism and improve the efficiency of the experiment (so far limited to <10 % of the theoretical limits, even at a moderate 10 T field). This step is critical to the successful extension of the approach to higher magnetic fields (20 T and above).
Along with a few other labs, we recently questioned the way to evaluate/measure the efficiency achieved with a MAS-DNP experiment. We performed thorough studies to determine and quantify all the various parameters that need to be accounted for upon radical doping. This work led us to conclude that the use of the ubiquitous eON/OFF, the ratio of the signal with microwave irradiation (signal ON) and without microwave (signal OFF), was not an appropriate way to measure DNP efficiency since concurrent signal losses were not taken into account by this simple parameter. We thus advocate the use of sensitivity measurement (rather than eON/OFF) and accordingly introduced the Absolute Sensitivity Ratio (ASR) parameter as a convenient way to quantify the time-savings afforded by DNP. The use of this simple parameter allows to properly optimize sample preparation protocols since it fully takes into account the changing spin relaxation times, different experimental conditions, and influence of paramagnetic effects, among other factors. The use of this parameter was crucial for the successful implementation of Matrix-Free sample preparation protocols – which did not necessarily gave the highest eON/OFF but clearly returned the best overall sensitivity.
Complementing this work, we then introduced a new and simple way to correctly quantify polarizing agent efficiency (Coll. Prof. Vega, Weizmann Institute of Science). Our work showed that most of the current “gold-standard” biradical polarizing agents have not been as efficient as originally presumed. Through this study we explained, based on quantum mechanical simulations, the origin of this discrepancy, highlighted that this effect is biradical dependent, and that the DNP efficiency decreases with the sample spinning frequency for all the current bi-nitroxide radicals.